ext4fs error "ext4_mb_generate_buddy:741:group 16, 8160 clusters in bitmap, 4064 in gd" (with repro)

Here is how to reproduce it.  It happens during fstrim.  I found other
occurrences of the error in the mailing list, but they were not related
to trim so they may be something different.

modprobe scsi_debug dev_size_mb=256 lbpws=1
dd if=/dev/zero of=/dev/sdb bs=1M      
fdisk /dev/sdb
 >> create a new partition accepting all defaults
fdisk -lu /dev/sdb|tail -1
 >> should show: /dev/sdb1     57      524285      262114+  83  Linux

mkfs.ext4 /dev/sdb1
mkdir test
mount /dev/sdb1 test
fstrim ./test

Here is the output in dmesg:

[140934.644166] EXT4-fs (sdb1): mounted filesystem with ordered data mode. Opts: (null)
[140941.562060] EXT4-fs error (device sdb1): ext4_mb_generate_buddy:741: group 16, 8160 clusters in bitmap, 4064 in gd
[140941.603066] EXT4-fs error (device sdb1): ext4_mb_generate_buddy:741: group 25, 8192 clusters in bitmap, 7934 in gd
[140941.613060] EXT4-fs error (device sdb1): ext4_mb_generate_buddy:741: group 27, 8192 clusters in bitmap, 7934 in gd
[140941.634074] EXT4-fs error (device sdb1): ext4_mb_generate_buddy:741: group 31, 8192 clusters in bitmap, 8159 in gd

Hope this helps,

Paolo